This invention relates to a method and apparatus for determining whether the burst signal of a color television signal is sampled at a predetermined phase angle and, more particularly, to a method and apparatus for using this determination to control the actual sampling phase. The present invention is particularly useful in digitally encoding a composite color television signal.
The technique of digitally encoding a television signal, and particularly a composite color television signal, is well known. Typically, sampling pulses are generated in synchronism with the color television burst signal, the sampling pulses having a repetition rate that is a multiple of the burst signal frequency. Each sample of the color television signal is encoded, or digitized, such as by pulse code modulation (PCM). Digitally encoded television signals are used in, for example, time base error correction devices, noise suppression devices, the addition of various special video effects, and the like. Digitized video signals are particularly advantageous for recording/reproduction, and also for special types of transmission.
In many television transmission systems, such as the NTSC system, the PAL system, and the like, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. Since the phase of the color subcarrier signal thus is used to represent color information, it is important that, when encoding the color television signal in digital form, the phase of the sampling pulses be accurately controlled. Undesired phase shifts, such as may be due to temperature drift, aging of the electrical components, and the like, may result in a phase error in the sampling pulse relative to the chrominance subcarrier signal which has the effect of distorting or interfering with the overall chrominance effect of the video picture which ultimately is reproduced from the digitized video signal.
To identify the aforementioned phase shifts, or errors, between the sampling pulses and the chrominance subcarrier signal, the instantaneous phase angle of the usual burst signal at the time of sampling is determined. If the phase angle then exhibited by the burst signal at the sampling time differs from a desired phase angle thereof, the phase of the sampling pulses may be adjusted accordingly. However, the number of samples which are needed to determine the sampled phase angle of the burst signal generally requires a relatively complicated calculating circuit, a memory device of high storage capacity, and necessitates a complex operation.